This invention relates to a shadow mask for a color picture tube, and more particularly, to a method for preparing the shadow mask.
A color picture tube, as shown in FIG. 1, generally comprises a glass envelope 1, in-line electron guns 3 emitting three electron beams 11R, 11G and 11B, and a phosphor screen 5 containing red, green and blue phosphor dots (not shown) which emit visible light when excited by the electron beams 11R, 11G and 11B. Electron guns 3 are located in a neck portion 2 of the envelope 1, while the phosphors, arranged in dotted shapes of cyclically repeating colors, are coated on the inner surface of the panel portion 4 of the envelope 1. Connecting neck portion 2 with the panel portion 4 is a funnel portion 12. The electron beams 11R, 11G and 11B are deflected by magnetic fields produced by a deflection yoke (not shown) surrounding a portion of the neck portion 2.
Near the screen 5 is a shadow mask 6 having a plurality of circular apertures 10 shown in FIG. 2 corresponding to the position of the phosphor dots. The shadow mask 6 is attached to a mask frame 7 supported within the envelope 1 by frame holders 8 which are releasably mounted on panel pins 9 embedded in side walls of the panel portion 4.
The shadow mask 6 has the function of causing the electron beams to accurately bombard the phosphor dots. Therefore, the sectionl shape of the apertures 10 is carefully designed. Namely, as shown in FIG. 2, to form the apertures 10, the shadow mask 6 has a front opening 13a facing toward the phosphor screen, a rear opening 13b facing toward the electron guns and an inner wall 13c connecting the front opening to the rear opening. The inner wall 13c has a most constricted portion 13d between the openings 13a and 13b to determine the spot size of the electron beam 11B projected on the phosphor screen.
Further, to avoid undesirable bombardment of the electron beam 11B on the inner wall 13c, the front opening 13a is larger than the rear opening 13b, while the inner wall 13c is inclined. In the shadow mask, if the position between the front and rear openings 13a and 13b and size of the most constricted portion 13d are shifted from designed value, the amount of electron beam passing through aperture 10 will change and thus the picture quality will be deteriorated. Also, if the electron beams 11B, 11G and 11R bombard the inner wall 13c, the color purity of the picture will be deteriorated due to reflected electron beams.
This type of shadow mask has been manufactured by photolithography. A typical process for manufacturing the shadow mask is disclosed in U.S. Pat. No. 3,973,965. Namely, as shown in FIG. 3A, a pair of photosensitive layers 14 are formed on both surfaces of a strip-shaped metal sheet 15 by coating photosensitive resin liquid on both surfaces, after the surfaces have been washed and dried. A pair of negative films 16a and 16b having dot patterns of different sizes formed thereon, are put on the photosensitive layers 14. Then, the photosensitive layers 14 are exposed to light emitted from light sources 17 through the negative films 16a and 16b, respectively (FIG. 3B). Next, the exposed photosensitive layers 14 are developed. Then, the unexposed portions of the layers 14 are removed. Thus, a pair of resist films 18a and 18b, which are composed of dot-shaped opening patterns corresponding to the dot patterns of the negative films 16a and 16b are formed on the sheet 15 (FIG. 3C). The resist films 18a and 18b are dried and baked to increase of the degree of corrosion resistance.
After this, an etching solution is sprayed on both surfaces of the sheet 15 to form the apertures 10. As a result of etching, large holes 19a growing from large openings 20a in the resist film 18a and small holes 19b growing from small openings 20b in the resist film 18b are linked to each other (FIG. 3D). The large openings 20a are larger than the small openings 20b. Finally, the resist films 18a and 18b are peeled off and removed from the surfaces (FIG. 3E).
During the etching process mentioned above, lateral extensions 21a and 21b of the resist films 18a and 18b are inevitably formed because of the lateral etching beneath the resist films which accompanies the growth of the holes 19a and 19b, as shown in FIG. 4. In FIG. 4, the lateral extension 21a of width d results from the isotropic etching. The extension 21a is destroyed and peeled off due to the pressure of spraying of the etching solution. Consequently, the configuration of the apertures is altered from the design configuration.
The lateral etching mentioned above is liable to occur in thick shadow masks, which are used when the shadow mask is required to have a high mechanical strength, for example, with flattered shadow masks. For instance, when the sheet thickness is increased to 0.3 mm from a thickness of 0.15 mm used for a normal shadow mask, the etching time increases approximately 3 times. Thus, the lateral etching progresses in proportion to the progress of etching. Consequently, the lateral extension of the resist film increases and, finaly it is peeled off.
Also, the lateral etching is liable to occur in a high-resolution color picture tube shadow mask having small-sized apertures and a smaller array pitch compared with ordinary shadow masks. Namely, since it is hard to circulate the etching solution in the aperture due to the small openings, the etching time increases compared with that of an ordinary shadow mask.